Intent to implement and ship: PutForwards annotations on various style rule properties

[Boris Zbarsky]

This is part of the work in
<https://bugzilla.mozilla.org/show_bug.cgi?id=851892>: Now that CSS
rules are on WebIDL bindings, we can do the [PutForwards] bits that are
in the spec draft at https://drafts.csswg.org/cssom/ for them.
Specifically, these:

1) On CSSStyleRule, [PutForwards=cssText] for .style.
2) On CSSImportRule, [PutForwards=mediaText] for .media.
3) On CSSMediaRule , [PutForwards=mediaText] for .media.
4) On CSSPageRule, [PutForwards=cssText] for .style.

I did spot-check Chrome and it has item #1 from the list above; I didn't
check the other items, nor in other browsers.

We already support [PutForwards=cssText] for .style on HTMLElement. We
do _not_ support [PutForwards=mediaText] for .media on stylesheets at
the moment, however.

I don't expect doing this to cause compat issues, but wanted to give
people a heads-up, especially because the spec draft seems to be in an
unknown state of stability.

-Boris